MAINTAINERS: net: update sfc maintainers

Add myself and Edward Cree as maintainers.
Remove Shradha Shah, who is on extended leave.

Cc: David S. Miller <davem@davemloft.net>
Cc: Edward Cree <ecree@solarflare.com>
Cc: Shradha Shah <sshah@solarflare.com>
Signed-off-by: Bert Kenward <bkenward@solarflare.com>
Acked-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>